In conventional continuous galvanising processes, steel strip, after being cleaned and otherwise conditioned for the adherent acceptance of the coating, is fed from above into a bath of molten zinc or zinc based alloy. The strip passes around a so called "sink roll" submerged in the bath, then emerges from the bath, and passes between coating thickness control devices, which return surplus liquid coating to the bath. The coating is then allowed or caused to solidify and the coated strip is finally coiled for storage, further processing or sale.
The sink roll, being submerged in the bath, operates in a hostile environment and thus is a source of trouble and unreliability unless carefully maintained. Even when adequately maintained, unavoidable wear and tear requires its periodic replacement. Furthermore, dross is sometimes dragged from the surface of the bath by the strip and may become attached to the sink roll and rough alloy growths tend to form on the roll's surface. That dross and those growths damage the strip requiring frequent shut down of the line for removal and replacement of the sink roll with a new or renovated roll. Thus it would be desirable to eliminate the sink roll.
With that desirability in mind, it has been proposed to provide at least one inlet opening in the pot, positioned below the normal operating level of the bath liquid so that a strip to be coated may enter the pot, either horizontally or from below, and depart, either through a similar exit opening or through the mouth of the pot, without need for a change in direction of the strip's pass line within the bath.
It is of course necessary to prevent the outflow of bath liquid through the opening or openings and various electromagnetic plugging means have been proposed for that purpose.
For descriptive convenience the surface of the liquid metal that is supported or otherwise restrained by forces generated by the electro-magnetic plugging means rather than by a solid component of the pot is referred to hereinafter as the "bare" surface of the liquid metal.
Prior proposed electromagnetic plugging means have usually fallen into either of two categories, namely those utilising either poly-phase energising windings or multiple pole electromagnets and switching devices which provide moving magnetic fields passing through the liquid or within the space into which the liquid might otherwise leak, and those which are analogous to electric motors utilising either permanent magnets or DC or single phase electro-magnets in combination with a transverse electric current. All such electro-magnetic plugging devices rely on the interaction of electric currents and magnetic fields, either generated independently or induced one by the other, and the currents are either DC or power frequency and the fields are likewise either steady or oscillating at power frequencies. In both categories of plugging means, the magnetic field and/or the electric current passes through the bath liquid adjacent to the opening to generate restraining forces therein.
Prior proposed electro-magnetic plugging means of the kind discussed above require relatively complex assemblies of components in close association with the liquid metal, thus they all operate in a hot and frequently crowded environment. This leads to design difficulties, limitations on the size and shape of the openings that may be plugged and low operating life expectancies.
Those prior proposed plugging means all suffer from operating deficiencies as well. For example, those which use DC currents flowing between electrodes in contact with the liquid metal, for example to generate a lifting force in the liquid to prevent it falling through a strip inlet opening in the floor of the pot, are inherently unstable. If an adventitious localised downwards projection forms in the bare surface of the liquid metal, then the current density in the projection becomes less than the average current density in the bare surface skin as a whole. Thus the upwards electro-magnetic force on the projection is reduced, and hydrostatic pressure causes it to grow. The more it grows the lower the restoring force becomes, until eventually the projection breaks away from the bare surface as a droplet of bath liquid. After breaking away there is no current through the droplet, the restoring force on it drops to zero, and the droplet falls through the opening. Indeed, in such plugging means any disturbance of the bare surface usually leads to a continuous rain of droplets from it, in that the break away of each droplet may cause sufficient disturbance to initiate the formation of another.
Also, all such prior known proposals that operate at power frequencies, produce flows of liquid within the bath adjacent the opening. Thus the bath is turbulent at the very position where it contacts the strip to be coated, this turbulence seriously degrades the surface quality of the coating on the finished product.
Finally, it should be mentioned that the devices under discussion have or produce powerful magnetic poles adjacent the opening, these interact with ferrous substrate strips tending to attract the strip towards the poles. Any adventitious deviation from the central pass line towards one pole and away from another causes the attraction of the strip towards the one pole to increase and that towards the other to decrease, so producing a deviating force which increases with increasing deviation. Thus the situation is clearly inherently unstable. In the event, successful operation with steel strip requires expensive guide rollers to be positioned closely adjacent to the opening, and an objectionably high tension to be maintained in the strip to prevent deviation, a tension which is not readily attained in practice.
In view of the foregoing and other deficiencies of what may be termed zero or power frequency plugging devices, such devices have not found widespread acceptance or use within the metal coating industry.
One other prior proposal has been suggested, namely the use of a high frequency, oscillating, but spatially stationary electro-magnetic field, positioned so as to exclude the bath liquid from the pot opening. This proposal relies on the fact that such a field generates high frequency eddy currents within the bath. Due to their high frequency the eddy currents flow only in a thin surface layer of the liquid (the so called and well known "skin effect"). The reaction between the surface currents and the field is one of mutual repulsion, and at sufficiently high frequencies the field is effectively excluded from penetrating the liquid. In those circumstances the field behaves as a resilient cushion that may be distorted or compressed by the bare liquid surface but resists penetration of the liquid into the space occupied by the field. The resisting force is perpendicular to the direction of the flux lines of the field and the bare surface of the liquid, and is proportional to the degree of distortion or compression of the field. Thus, unlike lower frequency interactions, the situation is inherently stable, in that an adventitious projection of the liquid surface into the field space produces a localised distortion of the field and an accompanying increased localised resistance to further intrusion.
Furthermore, in the absence of electric currents or electric-fields within the body of the bath liquid there is no turbulence induced in the liquid by the operation of the plugging means.
Thus, high frequency plugging means overcome the major deficiencies of zero or low frequency plugging means, but they are subject to their own inherent limitations.
In particular a high density magnetic field is required if sufficient force is to be generated normal to the bare liquid surface to resist the hydrostatic pressure at the bottom of a liquid metal bath of a depth sufficient to enable a reliable continuous strip coating operation to proceed. This in turn requires high energy generating coils and places a premium on the use of pot and pot opening shapes and dimensions that minimise the extent of the bare liquid area to be supported or otherwise restrained. This at least requires the strip inlet opening to provide only small clearances for the strip passing through it. This, in turn, requires precautions to ensure that the strip does not deviate from the intended pass line to any great extent.
A coating pot provided with a conceptually simple form of high frequency electro-magnetic plugging means is disclosed in Japanese patent No.04-099160 (Nippon Steel). In this instance the "pot" is a hollow, rectangular prismatic cell of silicon carbide some 100 mm wide with a slot some 20 mm wide in its floor. A steel sheet moves upwardly through the slot and through a galvanising bath contained in the cell. The lower part of the cell is surrounded by a solenoid coil that is energised at 20 kHz and has a vertical centre plane coinciding with that of the cell. The slot is set to one side of that centre plane. This is described as causing the lower part of the bath liquid to be pushed towards the centre plane of the cell clear of the slot but leaving the upper part of the bath unaffected. A steel strip to be coated is shown travelling upwardly through the slot and through the upper part of the bath.
Bearing in mind that the 20 kHz field would be effectively excluded from the bath and the strip, it is clear that the field of this device would be asymmetric with regard to the strip in a critical zone immediately above the slot. Indeed because of the shielding effect of the strip there would be very little if any field on the inside of the strip to push the liquid back from the inside of the strip immediately above the slot. This prior proposal would have three major deficiencies, (i) the above mentioned asymmetry would create substantial out of balance lateral forces on the strip requiring special arrangements and objectionably high tension in the strip to reliably maintain the strip out of contact with the walls of the narrow slot, (ii) the deficient field adjacent the inside of the strip would allow liquid to fall through the slot on the inside of the strip, and (iii) the large, bare surface area of liquid would require a large volume, high density field with consequent high power requirements for the generating coil. It might be thought that (i) and (ii) would be overcome by centralising the position of the slot. Indeed that may remove the out of balance force on the strip produced by the plugging field and may prevent leakage when a strip is present in the slot, but would still leave the strip free to move laterally from the pass line. More importantly the naturally densest part of the field within the bore of the coil is substantially vertical and so is not well oriented to provide a vertical restraining force. Thus, in the absence of a strip, restraint would depend on the relatively lower density diverging field at the top end of the solenoid. This would necessitate a very high power coil producing an unnecessarily dense field overall if leakage is to be efficiently prevented.